>     >     Perhaps, we should add a new user option with 3 possible values:
>     >     1. don't stop in the initial buffer
>     >     2. stop once in the initial buffer
>       3. stop in every buffer which contains the search string
> 	 (and always stop in the first buffer)
>
>     So there are three useful options (and let's try using the 3-rd as the
>     default).
>
> If you want to implement all three, please do.

I implemented a new user option isearch-buffers-pause with these
three values, with the default to pause in every buffer that contains
the search string.

Also I implemented a new minor mode isearch-buffers-minor-mode
that sets a group of variables necessary for this feature.
Turning off this minor mode deletes all created buffer-local bindings.
So any major mode that uses this feature can simply set the variable
isearch-buffers-next-buffer-function and call isearch-buffers-minor-mode.

The code implementing this feature has so many lines now that I think it
needs a separate file.  One possible name would be isearchb.el, but this
name is already occupied by another package.  So I think a good name will
be isearch-et.el (isearch everything :-) as I plan to add more isearch
extensions to this new file.

Below are two NEWS entries, a patch for the first package add-log.el
that uses this feature, a small patch for isearch.el, and a new file
isearch-et.el:

** The package isearch-et.el has been added.  It implements a new mode
`isearch-buffers-minor-mode' that allows isearch to search through
multiple buffers.  In this mode a new variable
`isearch-buffers-next-buffer-function' defines the function to call to get
the next buffer to search in the series of multiple buffers.

** isearch can now search through multiple ChangeLog files.
When running isearch in a ChangeLog file, if the search fails,
then another C-s tries searching the previous ChangeLog,
if there is one (e.g. go from ChangeLog to ChangeLog.12).

Index: lisp/add-log.el
===================================================================
RCS file: /sources/emacs/emacs/lisp/add-log.el,v
retrieving revision 1.197
diff -c -r1.197 add-log.el
*** lisp/add-log.el	10 Oct 2007 18:52:44 -0000	1.197
--- lisp/add-log.el	20 Oct 2007 00:00:55 -0000
***************
*** 760,766 ****
         'change-log-resolve-conflict)
    (set (make-local-variable 'adaptive-fill-regexp) "\\s *")
    (set (make-local-variable 'font-lock-defaults)
!        '(change-log-font-lock-keywords t nil nil backward-paragraph)))
  
  ;; It might be nice to have a general feature to replace this.  The idea I
  ;; have is a variable giving a regexp matching text which should not be
--- 760,788 ----
         'change-log-resolve-conflict)
    (set (make-local-variable 'adaptive-fill-regexp) "\\s *")
    (set (make-local-variable 'font-lock-defaults)
!        '(change-log-font-lock-keywords t nil nil backward-paragraph))
!   (set (make-local-variable 'isearch-buffers-next-buffer-function)
!        'change-log-next-buffer)
!   (isearch-buffers-minor-mode))
! 
! (defun change-log-next-buffer (&optional buffer wrap)
!   "Return the next buffer in the series of ChangeLog file buffers.
! This function is used for multiple buffers isearch.
! A sequence of buffers is formed by ChangeLog files with decreasing
! numeric file name suffixes in the directory of the initial ChangeLog
! file were isearch was started."
!   (let* ((name (change-log-name))
! 	 (files (cons name (sort (file-expand-wildcards
! 				  (concat name "[-.][0-9]*"))
! 				 (lambda (a b)
! 				   (version< (substring b (length name))
! 					     (substring a (length name)))))))
! 	 (files (if isearch-forward files (reverse files))))
!     (find-file-noselect
!      (if wrap
! 	 (car files)
!        (cadr (member (file-name-nondirectory (buffer-file-name buffer))
! 		     files))))))
  
  ;; It might be nice to have a general feature to replace this.  The idea I
  ;; have is a variable giving a regexp matching text which should not be

Index: lisp/isearch.el
===================================================================
RCS file: /sources/emacs/emacs/lisp/isearch.el,v
retrieving revision 1.303
diff -c -r1.303 isearch.el
*** lisp/isearch.el	29 Aug 2007 05:28:05 -0000	1.303
--- lisp/isearch.el	20 Oct 2007 00:02:31 -0000
***************
*** 2035,2042 ****
  			   (if isearch-forward (< pos2 pos1) (> pos2 pos1))))
  	      (setq pos1 pos2)
  	      (set-match-data match-data)))))
!     (if pos1
! 	(goto-char pos1))
      pos1))
  
  (defun isearch-search ()
--- 2045,2059 ----
  			   (if isearch-forward (< pos2 pos1) (> pos2 pos1))))
  	      (setq pos1 pos2)
  	      (set-match-data match-data)))))
!     (when pos1
!       ;; When using multiple buffers isearch, switch to the new buffer here,
!       ;; because `save-excursion' above doesn't allow doing it inside funcall.
!       (if (and (boundp 'isearch-buffers-next-buffer-function)
! 	       isearch-buffers-next-buffer-function
! 	       (boundp 'isearch-buffers-current-buffer)
! 	       (buffer-live-p isearch-buffers-current-buffer))
! 	  (switch-to-buffer isearch-buffers-current-buffer))
!       (goto-char pos1))
      pos1))
  
  (defun isearch-search ()